Conventionally, a polishing pad is produced by using, as base material, a nonwoven fabric or textile fabric formed of synthetic fibers with synthetic rubber or the like, coating its surface with a polyurethane based solution, solidifying the polyurethane based solution by a wet solidification technique to form a porous surface layer having continuous air holes, followed by grinding or removing the top surface of the surface layer as required (see patent document 1). In this polishing pad, the ground surface of the polishing pad is formed only of a porous polyurethane layer with none of the fibers constituting the base material exposed from the surface.
This type of polishing pads have already been in wide use as polishing pads for precision polishing of surfaces of electronic components including liquid crystal glass, glass disk, photomask, silicon wafers, and CCD cover glass. A polishing pad used for precision polished is required to have high accuracy in terms of aperture diameter variation and flatness (surface irregularities) in porous portions in the surface. In recent years, however, as sophisticated measuring instruments for fine-polished surfaces are developed and at the same time increased quality is demanded by users, that are increased needs for polishing pads that can meet demands for higher-precision polishing.
Conventional polishing pads as described above include polishing pads that are produced by impregnating with a polyurethane elastomer solution a needle punched nonwoven fabric formed of polyester short fibers with an average fiber diameter of 14 μm, wet-solidifying it in water, rinsing it, drying it, and buffing it to prepare a base material, followed by coating with a polyurethane solution and wet solidification (see patent document 1). However, it has been difficult for this technique to produce polishing pads that serve for polishing without causing defects such as scratches and particles on the mirror finished surfaces and serve for mirror-finishing an increased number of surfaces.
Aside from this, there is a proposal of a full grain leather-like sheet material formed of a grain layer of polyurethane combined with a base of a nonwoven fabric of polyurethane-containing ultrafine fibers with an average monofilament fineness of 0.001 dtex or more and 0.5 dtex or less (see patent document 2). The above proposal suggests that the material can serve as industrial tools such as polishing pads as one of its applications. However, the proposed full grain leather-like sheet material does not have an open polishing surface layer and does not have a uniform thickness, and it is difficult for such polishing pads to serve for polishing without causing defects such as scratches and particles on the mirror finished surfaces or serve for mirror-finishing an increased number of surfaces.